Blue Moons: What is a Blue Moon

What Is a Blue Moon?

Need a refresher on what a Blue Moon is? We get a lot of questions about the term, including:

• Is “Blue Moon” a scientific term used in astronomy?
• Did this term originate with Native American folklore, like a number of the other full Moon names?
• Does a Blue Moon really look blue?

In truth, the answer to all of these questions is “no.”

The modern understanding of “Blue Moon” only took off in the 1980s. It was a result of a much earlier mistake printed in a 1946 issue of Sky & Telescope magazine, and since then, the term has gone viral in the media.  

Next Blue Moon on August 30, 2023

The next Blue Moon will occur on Wednesday, August 30, 2023. 

This Blue Moon is also special because it’s the nearest supermoon of 2023. This means that this year’s Blue Moon is also the closest and brightest Moon of the year!  

A Blue Moon and a Super Moon occuring on the same night happens roughly every 10 years, and won’t occur again for another 14 years, in the night sky in January and March 2037.

A Supermoon is when the Moon is at or near its closest point to Earth in its oval-shaped orbit. A super moon can also cause higher tides than usual.

Read more about the August full Moons here.

Two Types of Blue Moons

There are two definitions of the term though we commonly use the second term (a second full moon in one month).

• Seasonal Blue Moon: The extra full Moon that occurs within an astronomical season. One season—defined by the dates of the solstices and equinoxes—typically has three full moons occurring within it. If a season instead has four full moons, then the third full moon (not the fourth) in the season may be called a Blue Moon. 
• Calendrical Blue Moon: The second full moon to occur in a calendar month. It takes our Moon about 29.5 days to complete one cycle of phases (from new Moon to new Moon), so if a full Moon occurs on the first of a month, there will be a second full Moon—a Blue Moon—at the end of the month, too (except in February).

Although the latter definition is the one more commonly followed today, the former actually came first. As mentioned above, a misinterpretation of the seasonal definition in the 1940s gave way to the calendrical definition, which was later popularized in the 80s and has stuck around to today.

How Often Does a Blue Moon Occur?

Most months have one full Moon, not two.

Since the Moon’s period of phases is 29 1/2 days, while months usually have 30 or 31 days, it’s obvious that if a full Moon lands on the first day of any month except February, it will repeat again at the end. 

Turns out, calendrical Blue Moons happen every 30 months on average. Two and a half years. Seasonal Blue Moons happen at a similar rate: about once every two to three years. So maybe “Once in a Blue Moon” isn’t so rare after all! 

Two Blue Moons in a Single Year

How often do we have two Blue Moons in a single year? (As in 2018, when there were Blue Moons in both January and March and no full Moon in February.)

This won’t happen again until 2037, when we will again have another Blue Moon in both January and March.

Learn More

Want to learn more about the Moon? See the Almanac’s Moon Phase Calendar and Full Moon Calendar!